1. Field of the Invention
The present invention relates generally to video signal decoders and, more specifically, to methods and systems for comb filtering composite signals to separate luminance and chrominance.
2. Related Art
Generally, video pictures or video signals are made up of video content signals, horizontal sync pulses and vertical sync pulses. Typically, a video picture includes a number of video frames. For example, according to the NTSC (National Television System Committee) format, there are 30 frames per second, and according to the PAL (Phase Alternation by Line) format, there are 25 frames per second. At the end of each frame, a vertical sync pulse is transmitted which indicates to a recipient electronic device that the frame has come to an end. Further, each video frame is made up of lines. In NTSC, there are 525 lines per frame, whereas, in PAL, there are 625 lines per frame. Each point in the line reflects the intensity of the video signal. At the end of each line, a horizontal sync pulse is transmitted which indicates to the recipient electronic device that the line has come to an end, so that the electronic device gets ready for the next line.
The screens of most monitors of electronic devices are drawn in a series of lines, left to right and top to bottom. When the monitor finishes drawing one line and reaches its right-most excursion, the beam is turned off while the monitor returns the beam to the left side of the screen. A similar process occurs when the last line on the screen is finished drawing, in which event, the beam traverses to the top left corner of the screen. In a video picture, the beam is moved to the left of the screen and to the top of the screen in accordance with and based on the detection of the synchronization signals. In other words, when the vertical sync pulse is detected the beam moves to the top left corner of the screen to begin drawing the next frame, and when the horizontal sync pulse is detected, the beam moves to the left of side of the screen to begin drawing the next line. Accordingly, it is quite important to properly detect the sync pulses.
Due to limitations on available bandwidth and the increased demand to transmit additional information on existing bandwidth, it is often necessary to multiplex or combine two or more information signals into a single composite signal.
A color television signal is an example of a composite signal. A color television signal comprises a luminance (brightness) component and a chrominance (color) component. These components are often represented as Y and C components wherein Y represents the luminance component and C represents the chrominance component.
Originally, broadcast television in the United States began with black and white broadcast and therefore lacked the chrominance component, C, of modern television's composite signal. Television standards and technology required that the black and white television signal, that is, the luminance component (Y), reside within 6 megahertz (MHz) of bandwidth space.
Eventually, however, technology advanced to provide color television. To allow black and white televisions to receive the new color signal broadcast, the color signal standard located the color information within the same 6 MHz of bandwidth space allotted to each channel of the black and white signal. Under this standard, the color information overlaps with the luminance information.
FIG. 1A illustrates a composite television signal on a coordinate system in which the horizontal axis 100 represents frequency and the vertical axis 102 represents amplitude. Signal line 104 represents the luminance information (Y) while line 106 represents the chrominance information (represented as I and Q) of the composite signal. As shown, the frequencies of these signals 104, 106 overlap. In an NTSC (National Television Standards Committee) system, the luminance information occupies the range DC to 4.2 MHz of bandwidth while the chrominance signal is bandlimited to the range approximately 0.6 to 1.3 MHz and is modulated onto a carrier at 3.58 MHz. The audio portion of the signal is at 4.5 MHz. While these two data signals conveniently fit within the 6 MHz of bandwidth space they are allotted, obvious decoding challenges are presented in order to separate the luminance information from the chrominance information.
FIG. 1B illustrates video signal line 120, which includes horizontal front porch 122, horizontal sync pulse 124, horizontal back porch 128, color burst 126 and horizontal active pixels 130. As shown, video signal line 120 begins at the falling edge of horizontal sync pulse 124 and ends at the falling edge of the next horizontal sync pulse. Horizontal front porch 122 is the period of time between the previous horizontal active pixels (not shown) and the beginning of horizontal sync pulse 122. Horizontal sync pulse 122 is a change in voltage of the video signal, which triggers the electronic device to stop the rightward progress of drawing the beam and begin drawing on the left side of the screen. Thus, each line begins with the start of the horizontal sync pulse and ends with the start of the next horizontal sync pulse. Horizontal back porch 128 is the period of time between the end of horizontal sync pulse 124 and the beginning of horizontal active pixels 130. According to NTSC and PAL formats, horizontal back porch 128 also includes color burst 126, as a color calibration reference.
The first decoding scheme adopted to separate the overlapping luminance (Y) and chrominance (C) signals comprises simple notch filtering in combination with band pass filtering. FIG. 2 illustrates a block diagram of a basic notch filter 152 and band pass filter 154. An incoming composite signal on line 150 is presented to both of the notch filter 152 and the band pass filter 154.
FIG. 3 illustrates the frequency response of a notch filter 152 and a band pass filter 154. The output of the notch filter generally comprises the luminance portion 174 of the composite signal while the output of the band pass filter generally comprises the chrominance portion 176 of the composite signal.
In particular, for NTSC video, the notch filter removes a portion of the composite signal centered at 3.58 MHz, but allows the remainder 174 to pass. While the notch filter 152 allows the majority of the luminance information 174 to pass, it undesirably removes components of the luminance signal having frequencies within the range of the notch filtered frequencies 177. The notch filtered frequencies that are removed range from 2.5 to 4.5 MHz. Stated another way, the notch filter allows the frequency band below 2.5 MHz and the frequency band above 4.5 MHz to pass.
The band pass filter 154 configured to operate in accord with the NTSC standard video allows a 2 MHz portion of the composite signal centered at 3.58 MHz to pass while removing portions outside of this band. This portion of the composite signal contains all the chrominance information. Undesirably, however, the output of the band pass filter also contains luminance components having frequencies within the band pass filter's frequency band.
Notch and band pass filtering suffers from numerous drawbacks as can easily be understood with reference of FIG. 3. In particular, the band pass filtered chrominance portion of the composite signal also contains luminance information, i.e., band pass filtering does not remove all luminance information from the chrominance signal. The unwanted luminance information in the chrominance signal introduces artifacts into the video image. This is most noticeable in pictures that contain closely spaced black and white lines, such as when the video display is of person is wearing a herringbone jacket.
Likewise, notch filtering the composite signal to remove the chrominance information from the composite signal to obtain the luminance information removes valuable portions of the luminance signal. A loss of luminance information is especially critical due to the human eye's sensitivity to brightness and contrast variations in a projected image.
FIG. 4 illustrates a conventional basic comb filter. In operation, a composite signal arrives at input 230 and branches into a line store 232, a first summing point 234 and a second summing point 236. The line store delays the incoming composite signal for a time equivalent to the period of one line. Further, some methods and systems utilizing comb filters for YC separation are described in U.S. Pat. No. 6,175,389, entitled “Comb Filtered Signal Separation”, issued Jan. 16, 2001, which is hereby incorporated by reference in its entirety.
In regions of the video image where the Y, I, and Q components on one line are the same as the previous line, the composite signal for one line is similar to the composite signal for the previous line. However, differences in phase do exist. The luminance portion of the composite signal is the same for both lines, but the chrominance portion of the composite signal for one line is phase shifted by 180 degrees compared to the chrominance portion of the composite signal for the previous line. This occurs because the subcarrier is phase shifted by 180 degrees relative to the previous line. With reference to FIG. 4, these phase differences and the additive and subtractive properties of the feed around loop cause the unwanted portions of the chrominance and luminance signals to cancel. The luminance signal is provided on a line 238 and the chrominance signal provided on a line 240.
Conventionally, comb filters have a frequency response configured to filter out a particular repeating frequency pattern in signals that are offset in time. The frequency response of a comb filter is illustrated in FIGS. 5A and 5B. The horizontal axis 250 represents frequency and the vertical axis 252 represents filter response in dB. As shown in FIG. 5A, the comb filter frequency response for the luminance portion (Y) of the signal is represented by line 254. Similarly, FIG. 5B illustrates the comb filter frequency response for the chrominance portion (C) of the signal, represented by line 256. In effect, the comb filter acts as a notch filter with a plurality of combs or teeth centered on or aligned with the frequency components of the desired signal. Because of its configuration, the comb filter excels in situations where the composite signal is stable or consistent from line-to-line such as, for example, in an area of solid color. Undesirably, however, at portions of a signal representing vertical transitions between colors or areas of motion in the video image, the comb filtering technique is undesirable, because conventional comb filtering creates artifacts at vertical transitions between differing colors and adjacent to moving objects.
A further drawback of utilizing conventional comb filters is that such filters are sensitive to imperfections in the line-to-line subcarrier phase difference. This sensitivity results from the line store 232 and the summing points 234, 236 in the comb filter. Subcarrier phase differences occur when the phase between the current signal and a line stored signal is other than 180 degrees out of phase. In such a situation, rather than perfectly adding or canceling at the summing points 234, 236, the signals, being out of phase, combine inaccurately and provide inaccurate luminance and chrominance signals.
Therefore, there is a need for methods and systems for a more robust separation of luminance and chrominance components of composite signals without great additional cost and complexity.